1. Field of the Invention
This invention relates to a semiconductor photo detector for use in a line image sensor for image reading of a copying machine, facsimile or the like, or for use in a two-dimensional image sensor for image input of a video camera or the like, particularly to a semiconductor photo detector making use of an avalanche effect for amplifying optically-formed carriers by impact ionization.
2. Description of the Related Art
As a device for reading light in a visible light range, CCD has been widely employed. A thin-film type image sensor using a semiconductor thin-film has also been proposed and it has already been industrialized in some fields. These photo detectors make use of a photo diode as a light-sensing part. They form, in principle, one or less than one electron for one light quantum and have no amplification effect. In general, it has been widely employed to equip a photo detector with an external amplification circuit, by which amplification of electrons is conducted to improve the sensitivity. According to this method, a noise component in the photo detecting part is also amplified at the same time, which inevitably leads to the lowering in the SN ratio. Such a detector is therefore accompanied with the drawback that in order to obtain a clear image using the detector, image pickup should be conducted after applying strong light to the object to be read for preparing the conditions under which sufficient reflected light is available.
With a view to overcoming the above-described drawback, a detector capable of conducting highly-sensitive image pickup by employing a semiconductor film made of crystal Si, Se or the like and imparting the photo-detecting part with an amplification effect has been industrialized in recent years. In this detector, a high electric field is applied to a semiconductor film made of crystal Si, Se or the like, whereby avalanche amplification (avalanche effect) is conducted. A Photo diode making use of an avalanche amplification effect (said diode will hereinafter be abbreviated as "APD") is now attracting attentions as a highly-sensitive semiconductor photo detector which can detect feeble light.
As this APD, there exists, as illustrated in FIG. 12(a), a single crystal Si pin APD comprising an n.sup.- electrode 201 made of silicon to which impurities have been doped, an SiO.sub.2 layer 202, an n.sup.+ layer 203, a p layer 204 which will be an avalanche region, a p.sup.- layer 205 which will be an optical absorption region, a p.sup.+ substrate 206 and a p.sup.- electrode 207 made of silicon to which impurities have been doped.
FIG. 12(b) is a schematic view illustrating the band structure of the above-described APD at the time when reverse bias is applied. The incident light irradiated from the side of the n.sup.- electrode 201 is absorbed by the p.sup.- layer 205 (which will be an optical absorption layer), whereby photoelectric transfer is conducted. Of an electron-hole pair formed in the p.sup.- layer 205, the electron travel toward the n.sup.- electrode 201 and the hole travel toward the p.sup.- electrode 207, respectively. The p layer 204 (which will be a carrier multiplication layer) has a strong electric field so that there appears an avalanche phenomenon, that is the phenomenon forming a large number of electron-hole pairs by the impact ionization during traveling of electrons, leading to the occurrence of a multiplication effect for forming a plurality of electron-hole pairs per one photo quantum.
The multiplication factor at this time depends on the ionization rate .alpha. of electrons. The larger the .alpha. is, the higher multiplication factor can be obtained. The term "ionization rate .alpha." as used herein means the number of electron-hole pairs formed at the time when one electron travels for a unit distance by impact ionization. The ionization rate d shows an exponential increase with an increase in the strength of the electric field so that the larger multiplication factor can be obtained by increasing the electric field.
The single crystal Si pin APD has been industrialized as a highly-sensitive semiconductor photo detector which has sensitivity to a range of from visible light to near infrared light (.lambda.=0.45-1.0 .mu.m) and can detect even feeble incident light. It is, however, accompanied with the following drawbacks:
(1) it requires a high driving voltage (.about.100V) because a high electric field is applied by the externally applied voltage to cause impact ionization of carriers; PA1 (2) owing to the operation in the high electric field, leakage current (dark current) generated at the time when no light is irradiated is large; and PA1 (3) avalanche amplification is accompanied with the occurrence of noise (excess noise), which lowers a signal to noise ratio (SN ratio).
According to the report of R. J. McIntyre in "IEEE Transactions Electron Device, 13, 164(1966)", it has been elucidated that when the ionization rate of electrons and that of holes are designated as .alpha. and .beta., respectively, the excess noise generated during the above-described avalanche multiplication depends on the ratio of these ionization rates (impact ionization coefficient ratio) k=.beta./.alpha., and in order to decrease the excess noise, the ratio k may be lowered for the electron multiplication while it may be raised for the hole multiplication, in other words, only the ionization rate of one of the carriers (electron or hole) to be multiplied may be increased.
In the case of single crystal Si, the ionization rate d of electrons is much larger than the ionization rate d of holes so that it is necessary to increase only the .alpha. to decrease the excess noise. In the single crystal Si pin APD, however, the ionization rate .alpha. of electrons and ionization rate .beta. of holes are determined according to the electric field strength of the avalanche region so that it is impossible to control the values of .alpha. and .beta. independently and the larger the electric field strength, the larger the value of k. In other words, as the electric field strength is heightened to obtain a larger multiplication ratio, the excess noise increases, inevitably leading to the reduction in an SN ratio.
The above-described report further describes that when only one of the carriers is multiplied, the excess noise index F becomes 2. In the case of ideal, noise-free multiplication, the index F may be 1 so that there remains somewhat noise generating mechanism in the above case. It is considered as the generating mechanism that the place where ionization occurs fluctuates within a semiconductor photo detector so that the whole multiplication factor fluctuates, in other words, the fluctuation becomes a noise source. It is considered that to suppress the fluctuation and to obtain a higher SN ratio, specification of the place where ionization occurs within the detector is effective.
With a view to overcoming the above-described problems of the single crystal Si-base pin APD, APD using a super-lattice structure of an amorphous Si semiconductor is proposed in "IEEE Trans. Electron Devices, 35, 1279(1988)". A description will next be made of this APD, with reference to FIGS. 13(a)-(C).
The APD using a super-lattice structure of an amorphous Si-base semiconductor comprises, as illustrated in FIG. 13(a), a transparent electrode 302 made of ITO, a p.sup.+ a--Si:H layer 303, a super lattice layer 306 serving as both an optical absorption layer and a carrier multiplication layer, an n.sup.+ a--Si:H layer 307 and an electrode 308 formed of Al, all of them being stacked one after another on a glass substrate 301. The super-lattice layer 306 is formed of an a--Si:H layer 304 which will be a well layer and an a--SiC:H layer 305 which will be a barrier layer, said layers being stacked alternately to be 10 layers in total. Concerning the p.sup.+ a--Si:H layer 303 and the transparent electrode 302, and the n.sup.+ a--Si:H layer 307 and the electrode 308, each pair is constructed to form an ohmic contact.
FIG. 13(b) is a schematic view illustrating the band structure of the above-described APD at the time when no voltage is applied. In the diagram, discontinuous amounts of the energy band of the conduction band and valence band in the hetero junction between a--Si:H and a--SiC:H are expressed by .DELTA.Ec and .DELTA.Ev, respectively. Concerning the band discontinuous amount in the a--Si:H/a--SiC:H hetero junction, that of the conduction band is larger, and .DELTA.Ec is 0.35 eV and .DELTA.Ev is 0.10 eV.
FIG. 13(c) is a schematic view illustrating the band structure of the above-described APD at the time when reverse bias is applied. The incident light from the side of the p.sup.+ a--Si:H layer 303 is absorbed by the super-lattice layer 306, whereby optoelectric transfer is conducted and a pair of electron and hole is formed. The electron and hole so formed travel toward the n.sup.+ a--Si:H layer 307 and the p.sup.+ a--Si:H layer 303, respectively. When the electron accelerated by the electric field enters into the well layer 304 from the barrier layer 305 of the super lattice layer 306, its energy condition becomes higher by .DELTA.Ec, that is, the band discontinuous amount of the conduction band, which heightens the ionization rate d of the electron in proportion. Repetition of the above-described procedure of the electron increases the number of the carriers.
In the case of the hole, on the other hand, no such phenomenon occurs because the band discontinuous amount .DELTA.Ev of the valence band is small. According to the above-described APD structure, only the ionization rate .alpha. of the electron can be increased and furthermore, the place where ionization occurs can be specified at the hetero junction part so that high sensitivity and low excess-noise properties can be attained. In addition, carriers receive energy by the band offset of the hetero structure so that the electric field strength necessary for the ionization of carriers can be reduced, which enables low voltage drive.
In the report by Sawada et al., in "Annual Meeting Preprint of the Television Society, 1995, p73", described is an APD having a graded super-lattice structure in which in an a--Si:H/a--SiC:H super lattice, a barrier layer has a saw-tooth potential structure. A description will next be made of this APD with reference to FIGS. 14(a)-(c).
The APD having a graded super-lattice structure is formed of an i-type a--Si:H 402, a graded super-lattice layer 405 serving as an optical absorption layer and a carrier multiplication layer, i-type a--Si:H 406, a p-type semiconductor layer 407 and a transparent electrode 408 made of Au, all of them being stacked one after another on an n-type single crystal Si substrate 401. The graded super-lattice layer 405 is constructed of an i-type a--Si:H layer 404 which will be a well layer and an i-type a--Si.sub.1-x C.sub.x :H (x=0-1) layer 403 which will be a barrier layer, said layers being stacked alternately to 6 layers in total.
FIG. 14(b) is a schematic view illustrating the band structure of the above-described APD at the time when no voltage is applied. The band structure of the graded super-lattice can be changed to a saw-tooth structure by continuously changing the composition ratio of the a--Si.sub.1-x C.sub.x :H layer within a range of x=0-1 at the time when the i-type a--Si.sub.1-x C.sub.x :H (x=0-1) layer 403 is deposited as a barrier layer.
FIG. 14(c) is a schematic view illustrating the band structure of the above-described APD at the time when reverse bias is applied. The avalanche multiplication mechanism is fundamentally equal to that of the super-lattice APD illustrated in the above FIG. 13. In this diode, however, there does not exist an energy barrier against electrons at the hetero junction part in the traveling direction of electrons, which makes it possible to avoid cooling of electrons by which energy is lost at the time when electrons enter from the well layer 404 to the barrier layer 403, or to prevent electrons from not being taken externally as signals owing to the accumulation of electrons in the well layer 404. It is therefore possible to conduct a sensitivity increase and reduction in noise furthermore.
The APD which employs the super-lattice structure of an amorphous Si-base semiconductor is however accompanied with the problem that it uses an amorphous semiconductor as the carrier multiplication layer so that electrons generated are trapped or form recombination in the film, leading to a large loss, and an amplification factor cannot be increased.
In the super-lattice APD of an amorphous Si base semiconductor, as the band discontinuous amount .DELTA.Ec of the conduction band is about 0.34 eV, smaller than the forbidden band width Eg (1.70 eV) of the i-type a--Si:H which is the well layer 404, it is necessary to apply a high electric field to the carrier multiplication layer in order to cause an avalanche phenomenon. By this application of the high electric field, electron-hole pairs are formed from the local level in the carrier multiplication layer, leading to the problems that large dark current is formed and a high SN ratio is not available.